Various methods are proposed to date as the noninvasive constituent concentration measuring method based on percutaneous irradiation of electromagnetic wave and/or observation of radiation. In these methods, an interaction between the objective blood constituent, for example, a glucose molecule in the case of a blood sugar level, and the electromagnetic waves having a particular wavelength, i.e., absorption or scattering is utilized.
However, the interaction between the glucose and the electromagnetic wave is weak, there is a limitation to intensity of the electromagnetic wave with which a living body can safely be irradiated, and the living body is a scatterer for the electromagnetic wave. Therefore, satisfactory result is not obtained so far in the blood sugar level measurement of the living body.
An photoacoustic method of irradiating the living body with the electromagnetic wave to observe an acoustic wave generated in the living body deserves attention among the conventional techniques of utilizing the interaction between the glucose and the electromagnetic wave.
The photoacoustic method is a method of measuring an amount of molecule in the living body by measuring pressure of the acoustic wave. That is, when the living body is irradiated with a certain amount of electromagnetic wave, the electromagnetic wave is absorbed in the molecule contained in the living body, the acoustic wave is generated by a local heating of the region irradiated with the electromagnetic wave followed by thermal expansion, and the pressure of the acoustic wave depends on the amount of molecule absorbing the electromagnetic wave. Furthermore among the photoacoustic method, a method in which heat is generated in a local area irradiated with the light, and the thermal expansion occurs locally without thermal diffusion to generate the propagating and finally utilized acoustic wave, is called the direct photoacoustic method.
The acoustic wave is a pressure wave propagating in the living body, and the acoustic wave has a feature that it is less prone to scattering effect as compared with the electromagnetic wave. Therefore, the photoacoustic method is a noteworthy technique in the blood constituent measurement of the living body.
FIGS. 49 and 50 show configuration examples of the prior art for constituent concentration measuring apparatus in which the photoacoustic method is utilized.
FIG. 49 shows an example for the first prior art example in which a light pulse is used as the electromagnetic wave (for example, see Non-Patent Document 1). In this example, blood sugar, i.e., glucose is set as a measuring object in the blood constituent. In FIG. 49, a drive power supply 604 supplies a pulse-shaped excitation current to a pulse light source 616, the pulse light source 616 generates a light pulse having a duration of sub-microsecond, and a living body test region 610 is irradiated with the light pulse. The light pulse generates the pulse-shaped acoustic wave called a photoacoustic signal in the living body test region 610, and an ultrasonic detector 613 detects the photoacoustic signal to convert it into an electric signal proportional to the acoustic pressure.
A waveform of the electric signal is observed by a waveform observing apparatus 620. Since the apparatus 620 is triggered by a signal synchronized with the excitation current, the electric signal proportional to the acoustic pressure is displayed at a predetermined position on a screen of the waveform observing apparatus 620, and the signals can be integrated and averaged.
Amplitude of the obtained electric signal proportional to the acoustic pressure is analyzed to measure the amount of blood sugar level, i.e., the amount of glucose in the living body test region 610. In the example shown in FIG. 49, the sub-microsecond light pulses are generated in a repetition up to 1 kHz, averaged measurement for 1024 light pulses provides the electric signal proportional to the acoustic pressure. However, the sufficient accuracy is not obtained.
Therefore, an example of the second prior art in which a continuously intensity-modulated light source is used is disclosed to increase the accuracy. FIG. 50 shows a configuration of an apparatus of the second conventional example (for examples see Patent Document 1). In this example, the blood sugar is set as the main measuring object, and multiple light sources having different wavelengths are used to attempt a measurement with the high accuracy.
To avoid explanation from becoming complicated, the operation with the two light sources will be exemplified with reference to FIG. 50. In FIG. 50, the light sources having the different wavelengths, i.e., a first light source 601 and a second light source 605 are driven to emit continuous light beams by a drive power supply 604 and a drive power supply 608 respectively.
The light beams output from the first light source 601 and the second light source 605 are modulated by a chopper plate 617 which is driven by a motor 618 and rotated at the constant number of revolutions. The chopper plate 617 is made of an opaque material, a shaft of the motor 618 is positioned at the center of concentric circles, of which circumferences where the light beams of the first light source 601 and the second light source 605 pass respectively have mutually-indivisible numbers of apertures.
The light beams output from the first light source 601 and the second light source 605 are intensity-modulated by a mutually indivisible modulation frequency f1 and a modulation frequency f2, the light beams are combined by a coupler 609, and the living body test region 610 is irradiated by the combined light beam.
In the living body test region 610, the photoacoustic signal having the frequency f1 is generated by the light of the first light source 601, and the photoacoustic signal having the frequency f2 is generated by the light of the second light source 605. The photoacoustic signals are detected by an acoustic sensor 619 and converted into the electric signals proportional to the acoustic pressures, and frequency spectrum is observed by a frequency analyzer 621.
In the example, all the wavelengths of the multiple light sources are set at absorption wavelengths of glucose, and photoacoustic signal intensity at each wavelength is measured as the electric signal corresponding to the amount of glucose contained in the blood.
In this configuration, a relationship between the measured intensity of the photoacoustic signal and the glucose concentration measured from the separately collected blood are previously stored to measure the glucose amount from the observed value of the photoacoustic signal.
On the other hand, in health management and treatment, it is important to continuously perform the measurement while the constituent concentration measuring apparatus is carried around. Therefore, a portable type or wearable constituent concentration measuring apparatus is also developed. The following examples for the third and fourth prior arts are disclosed as the portable type constituent concentration measuring apparatus.
The third example shown in FIG. 51 is an example mounted on a eyeglasses handle that comes into contact with the back of an ear (for example, see Patent Document 2). In FIG. 51, both a light source 500 and an acoustic wave detector 541 are embedded in a contact surface of an apparatus body 540 with a living body 499. In the acoustic wave generated in the living body 499 by the irradiation light emitted from the light source 500, a part of the acoustic wave propagating backward is detected by the acoustic wave detector 541.
The fourth example shown in FIG. 52 is an example mounted on an erring (for example, see Patent Document 2). In FIG. 52, the apparatus body 540 comes into contact with the living body 499 from both sides, the light source 500 is embedded in one of the contact surfaces of the apparatus body 540, and the acoustic wave detector 541 is embedded in the other contact surface. In the acoustic wave generated in the living body 499 by the irradiated light emitted from the light source 500, a part of the acoustic wave propagating forward is detected by the acoustic wave detector 541.    [Patent Document 1] Japanese Patent Application Laid-Open (JP-A) No. 10-189    [Patent Document 2] JP-A No. 8-224228    [Non-Patent Document 1] Thesis (University of Oulu, Finland) “Pulse photoacoustic techniques and glucose determination in human blood and tissue”, (IBS951-42-6690-0, http://herkules.oulu.fi/isbn9514266900/, 2002)